1. Field of the Invention
The present invention relates to a small-sized cryostatic device for photodetectors.
2. Description of the Prior Art
So as to present the highest possible sensitivity to radiations, some detectors such as infra-red detectors must operate at a very low ambient temperature. To obtain this result, the detectors are generally mounted in cryostatic devices of the type comprising a finger brought to a very low temperature on which the detector(s) to be cooled are mounted and which is surrounded by a wall forming a sealed enclosure with said finger. The wall is provided on the part facing the detectors with a window made from a material transparent to the radiations to be detected. The enclosure is moreover evacuated, generally by means of a pumping nipple, for the purpose of avoiding thermal losses around the cold finger. In the prior art devices, the cold finger is conventionally made from glass, the external wall being made from glass or from metal. The use of glass gives rise to certain problems. Thus, because of its malleability when hot, the cryostatic device is easily deformed during sealing together of the two parts and it is difficult to obtain precise dimensions. In addition, machining and grinding of the glass are delicate operations. It is therefor difficult to construct at a lower price a cryostatic device whose external wall may serve as positioning face. Consequently, it is necessary to use additional parts for positioning the cryostatic device. Thus, usually, the cryostatic device is mounted in a precise mechanical part fitting on to the external wall of the cryostatic device properly speaking. However, this mechanical part increases the mass and the space occupied by the cryostatic device.
Furthermore, the detectors are connected to the processing system by conductors whose number depends on the number of detectors mounted in the cryostatic device. Now, the conductors must sealingly pass through the enclosure of the cryostatic device. This may be achieved in numerous ways either at the level of the sealing between the two parts forming the finger and the external wall or by passing directly through the glass or the metal of the external wall. In the case of passing at the level of the sealing, it is necessary to use an additional insulating part, generally made from polyimide, on which the conductors are etched. Furthermore, to avoid any risks of short-circuits, the gap between two conductors must be sufficient, which limits the number of output conductors as a function of the external perimeter of the finger. Moreover, in the case of passing through a metal wall, the metal conductor passes through a glass ball which insulates it from the wall. In fact, all these elements lead to increasing the size of the cryostatic device or in limiting the number of outputs.
Now, infra-red detectors in particular are used more and more in so-called "air-borne" equipments. So attempts have been made to reduce as much as possible the size and weight of the cryostatic device.